For Example, Japanese Patent Application Publication No. 2006-054832 published on Feb. 23, 2006 discloses a remote supervisory control system. The system includes terminal devices and a center device which are configured to communicate with each other. Each of the terminal devices is connected with at least one monitored device. Each monitored device is monitored or controlled based on information such as monitor, control or the like which is transmitted between the center device and each terminal device.
In another monitoring system including a center device and terminal devices, for example, the center device transmits a request to each terminal device by polling. Each terminal device returns a response to the center device. Thereby, the center device obtains information from each of the terminal devices, and also transmits information (e.g., control information) to each terminal device. However, in the configuration that each terminal device communicates with the center device through a router, the center device cannot transmit a request to each terminal device by polling.
In the aforementioned remote supervisory control system, the center device cannot transmit a request to any terminal device in the destination through a destination router. However, each terminal device transmits a request to the center device, and thereby the center device can transmit a response to the originating terminal device through the destination router.
In any configuration of them, a request is transmitted to each destination at constant intervals in general. For example, when monitor information is transmitted from each terminal device to the center device, the center device returns control information to each terminal device at constant intervals.
However, in these communication systems, even if a center device has information to be transmitted to a specific terminal device, it cannot transmit the information to the specific terminal device until obtaining a request from the specific terminal device. That is, delay occurs.
If the constant interval is shortened, delay time can be shortened. However, when the number of the terminal devices is increased, the constant interval is more shortened. Accordingly, it is necessary to raise the communication speed between the center device and each terminal device. In addition, the processing load of the center device is increased.